There is known a background sheet feeder that employs a so-called electrostatic attraction and separation method of separating a sheet from other sheets by attracting the sheet using static electricity.
The background sheet feeder includes an endless belt and a charging member. The endless belt is formed by a dielectric member provided to face the upper surface of a stacked sheet stack and moving in the sheet feeding direction. The charging member applies an alternating voltage to a surface of the endless belt to charge the surface of the belt and generate an electric field, which in turn generates an attraction force that separates the uppermost sheet from the stacked sheet stack.
In the sheet feeder, the endless belt is wound around a pair of eccentric rollers, each of which rotates about an eccentric shaft connected to a drive device such as a motor. The eccentric rollers are rotated by the drive device, thereby moving the endless belt reciprocally toward and away from the upper surface of the sheet stack. Each of the eccentric rollers is held at a home position at which a circumferential portion thereof most distant from the eccentric shaft is located at the top. At the home position, therefore, the endless belt is separated from the upper surface of the sheet stack.
In the thus-configured sheet feeder, the endless belt rotates when the eccentric rollers rotate upon start of a sheet feeding operation. As the endless belt moves, the surface of the endless belt is charged. At the same time, the endless belt moves downward from the home position in accordance with the rotation of the eccentric rollers, and approaches the upper surface of the sheet stack. Then, each of the eccentric rollers rotates to a position at which the circumferential portion thereof most distant from the eccentric shaft is located at the bottom and the uppermost sheet is electrostatically attracted to the endless belt owing to the attraction force generated by the electrical charge. Thereafter, the eccentric rollers further rotate to separate the leading end in the sheet feeding direction of the uppermost sheet from the endless belt owing to the curvature of one of the eccentric rollers. As the rollers continue to rotate, the uppermost sheet enters the space between conveyance guides and is then sent downstream in the sheet feeding direction on a conveyance path.
As well as the above-described method, the method of moving the endless belt and the sheet stack alternately toward and away from each other includes, for example, a method of vertically moving a belt unit, which holds the endless belt, via a wire using a drive device such as a solenoid or a motor, and a method of vertically rotating one of the paired rollers, around which the endless belt is wound, about the other roller as a hinge using the drive force of a motor.
It is to be noted that the sheet feeder is not limited to the configuration of moving the endless belt toward and away from the sheet stack. Alternatively, for example, the sheet feeder can be configured to move the sheet stack toward and away from the endless belt by vertically moving a bottom plate loaded with the sheet stack via a linkage mechanism or a rack and pinion mechanism using the drive force of a motor or the like.
In the sheet feeder that moves the endless belt toward and away from the sheet stack by vertically moving the belt unit via a wire using a drive device such as a solenoid or a motor, however, if the drive device is rapidly driven to improve productivity, i.e., to increase the number of sheets fed per unit time, the wire may sag in response to inertia at the time of rapid drive. The slack in the wire hinders reducing the time taken for the repeating operation of moving the endless belt toward and away from the sheet stack, thereby limiting any improvement in productivity. It is therefore desirable to perform a drive operation that does not cause the wire to go slack in response to inertia.
Further, in the sheet feeder that vertically rotates one of the paired rollers, around which the endless belt is wound, about the other roller as a hinge using the drive force of a motor, the motor generates relatively high rotational torque to rotate the endless belt. Further, the motor is driven in the forward and reverse directions to vertically rotate the one of the rollers, and forward drive and reverse drive of the motor are performed in every sheet feeding operation. This type of sheet feeder, therefore, increases the load on the motor, and thus hinders improvements in productivity. The same problem occurs in the sheet feeder configured to move the sheet stack toward and away from the endless belt by vertically moving the bottom plate loaded with the sheet stack, the motor generates sufficient drive force for vertically moving the entire weight of the sheet stack loaded on the bottom plate. That is, the motor for vertically moving the bottom plate generates relatively high rotational torque sufficient for vertically moving the entire weight of the sheet stack, and as in therefore this type of sheet feeder also increases the load on the motor, hindering productivity gains.